1. Technical Field
The present invention relates in general to noise suppression within a computer system and in particular to reducing the distribution of noise within a computer system. Still more particularly, the present invention relates to placement of a ferrous material onto both sides of a printed circuit board to suppress unwanted noise on transmission lines.
2. Description of the Related Art
Controlling electrical interference which results from spurious electrical signals has become critical in the design of computer systems. Ever increasing requirements on the performance of new computer systems have led to widespread utilization of noise filtering techniques. New, low voltage, high frequency logic families produce more noise and are more susceptible to noise than their predecessors. State of the art computer systems are ever increasing in operating frequencies. Consequently, interference due to noise often receives more emphasis and investigation than timing and low power design challenges.
Research and development on computer technology has continually produced smaller and more compact systems. However, the transmission of logic signals and power has not received the same amount of design attention received by other components of computer systems. Transmission line interfacing is a necessary and integral part of computer systems. Many problems arise when long data distribution lines are required to transport low voltage digital signals. As a result of utilization of low voltages and high frequency, a low level noise level can cause unacceptable disruptive interference.
Printed circuit boards are made from sheets of insulative material, such as polyamide, or other insulators usually made from polymer compounds. Generally, a thin layer of copper is attached to a polymer sheet. The thin layer of copper is etched away from the base insulating material to form transmission lines which are not electrically connected to each other. Each isolated transmission line can be called a "trace." Printed circuit board technology is well known and the circuit board manufacturing will not be discussed here in detail.
A single layer printed circuit board, generally, has 2 layers of copper, one on the top side and one on the bottom side. Currently, state of the art circuit boards implementing 64 bit data buses might utilize a 24 layer circuit board. The copper layer generally contains hundreds of traces. This extreme density places a significant amount of electrical noise in close proximity to sensitive circuits.
Advances in printed circuit board technology allow shrinking of the minimum distance between adjacent circuit traces. Advances have led to an increase in the coupling capacitance and mutual inductance from a trace to adjacent or neighboring traces. Due to the increase in the coupling capacitance, an oscillator signal or synchronization clock can adversely affect adjacent circuits. If an adjacent trace is quiet, capacitive coupling induces noise or an unwanted signal into a quiet trace. This has a determinental effect on sensitive circuits and induces failures into the system.
The criticality of noise susceptibility is often dependent on the type of logic circuits utilized in the design. Dynamic logic circuit families and their derivatives have gained wide spread acceptance. However, the voltage required to switch logic gates of dynamic logic circuits is very low. Currently, threshold voltages of transistors are as low as 0.1 volt. Hence, state of the art logic circuits trade noise margin to reduce power consumption and delay.
High frequency, low voltage designs have increased noise susceptibility, and thus require greater utilization of noise attenuation methods. Data failures due to noise are sometimes called noise failure. Noise failures can be more menacing than timing failure problems. This is due to the amorphous form of noise. Often, noise takes no definite form or distinct manifestation. Therefore, discovery of the source and consequently, remedy of the failure can be very difficult to accomplish. For most circuits, timing failure can be recovered by changing the clock speed and thus allowing more time for signal propagation. However, noise failures are caused by many phenomena such as capacitive coupling, input slope, circuit length and logic families. Noise failure is much more difficult to predict and to control, than other phenomena due to the interaction of these phenomena.
Decoupling capacitors are frequently utilized to attenuate noise in circuit board designs. While decoupling capacitors can provide adequate noise filtering at frequencies up to 75 MHz, their performance at high frequencies is dramatically reduced by the presence of circuit resonances. Circuit resonance can arise from the interaction of the decoupling capacitors and stray inductance. Many engineers have observed and labored over noise problems that arise unexpectedly from unique combinations of noise frequencies, PC board layouts and decoupling capacitors.
Commercial electrical products are required to pass mandatory United States and International electronic emission requirements in order to be sold in these respective markets. High frequency noise is often produced on, and conducted through a printed circuit board's power and data distribution bus to the external environment. A manufacturer must control electronic emissions to pass government requirements. Examples of devices which are required to pass governmental regulations are computers, AC adapters, facsimile devices, printers, and other communication devices.
Internal and external cable assemblies or transmission lines in computer equipment often act as miniature antennas and radiate noise into the environment. Voltages and noise currents transform into radiated electronic emissions when conducted through transmission lines.
Circuits frequently radiate electronic noise which originates from data bus switching and clock signals. Most oscillators or clocks operate at a determined frequency. When a transmission line reaches a fraction of the wavelength of the oscillation frequency, the transmission line acts as an antenna. Likewise, a susceptible circuit having a short length can become a receiving antenna. For instance, a frequency of 1 GHz has a wavelength of approximately 1 foot. A half wave antenna which would very efficiently absorb or induce a 1 GHz noise signal would be only 6 inches long. Many power lines and data lines are 6 to 12 inches long. Radiating noise commonly interferes with low voltage logic circuits which utilize transmission lines of short distances. This is particularly true when coupling occurs because transmission lines are a fraction of a wave length of a spurious radiation frequency. Transmission lines are present in every chip, every circuit board and virtually everywhere in a computer system.
Higher circuit board trace densities are required for larger data buses. Common mode noise failures have become prevalent in today's circuits. Common mode currents are currents which flow on more than one transmission line in the same direction. Emissions emanating from a product in the form of common mode current can create interference problems for other circuits and also cause failure of government emission requirements. For typical emission problems, common mode current flows on ground, signals, and voltage planes from a source, such as an oscillator, towards a connector which provides the input/output to the circuit board or the system.
Many computer noise suppression techniques place a ferrous core material on cables and conductors. Cables might have ferrite beads placed over individual wires or entire cables. Alternatively, ferrite material is some times placed over flat ribbon cable assemblies. Cabling requirements utilizing ferrous beads are costly. Assembly of cables is normally done by outside contractors who specialize in wire harnesses. Utilizing outside contractors and vendors requires purchasing effort, quality control, inspection and additional part numbers. Placement of ferrous beads or cores on cables requires special handling so the cores are not broken during shipping and handling. All of the foregoing tasks add to the assembly costs of computer systems, and in particular add to the final cost of a computer system.
Cabling with magnetic cores requires detailed cable routing. Strain reliefs must be utilized at frequent intervals. Ferrous material is heavy, therefore, vibration and gravity can fatigue cable insulation and pull on crimped or soldered connections. Continuous flexing of a cable at the edge of the ferrous material can strain harden the copper wires and eventually cause an open circuit. Additionally, cables can unplug due to the force of the ferrous core pulling on the connection during shipping or vibration.
Brittleness and vulnerability of ferrous cores to physical shock are inherent characteristics of ferrous. Most ferrous cores are placed in equipment that is subject to the shock and vibration of shipping, handling, and the installation processes. Ferrous material assembled to cables can move and potentially break when impacted by a hard object. When installed on flexible cable harnesses, ferrous cores of significant mass must be encapsulated by heat shrink tubing or otherwise protected and secured in place, also adding to the overall cost. Heavy ferrous cores attached to cables cause a multiplicity of difficulties.
Hence, there is a need for a low cost, high density, reliable power and data transmission system with suppressed noise, and in particular, common mode noise. Additionally, there is a need for economical mounting of ferrous cores in a computer system such that the ferrous cores are sufficiently retained to reduce construction costs and eliminate fatigue concerns. The present invention is directed at reducing the cost and increasing the reliability of a computer system and while providing noise suppression at the source of undesirable emissions.